As a general rule, a frame has rims each presenting an internal bezel forming a groove. The shaping of a lens in order to enable it to be mounted in the frame selected by the future wearer then consists in modifying the outline of the lens so as to match it to the frame.
With lenses for frames having rims, the shaping of the lens includes a final step referred to as beveling (or ridging) that consists in forming a ridge, commonly referred to as a bevel, that is generally of triangular cross-section on the edge face of the ophthalmic lens. The bevel is for engaging in the bezel formed in the rim or surround of the eyeglass frame in which the lens is to be mounted.
Conventionally, shaper means are constituted by a machine tool known as a grinder that possesses a set of main grindwheels together with means for blocking and driving the lens in rotation, which means are constituted by two rotary shafts on a common axis mounted to move axially relative to each other in order to clamp the lens on its axis between them.
In general, a roughing step is performed initially on the periphery of the lens by means of a cylindrical rouging grindwheel forming part of the set of grindwheels. During the roughing step, the lens is reduced approximately to the shape that it is desired to give to the lens. Thereafter, using a cylindrical finishing wheel of finer grain and provided with a V-shaped ridging groove, the finishing step is performed. The shape desired for the periphery of the lens is reached accurately and the bevel is formed by the ridging groove that imparts its shape to the edge face of the lens.
The shaped lens thus has a V-section bevel presenting two opposite conical flanks each extended to the corresponding front or rear face along the edge face of the lens by a cylindrical structure referred to as the foot of the bevel. Each foot on either side of the bevel is a substantially plane portion of the edge face of the lens that is situated beside the front or the rear flank of the bevel.
Nevertheless, when the lens is strongly cambered, it can be useful to encourage engagement of the bevel in the bezel of the corresponding rim by offsetting the height position of the feet of the bevel. To achieve such an offset in the height position of the feet of the bevel known as providing relief, it is necessary to provide a finishing grindwheel that presents a specific difference in diameter on either side of the ridging groove. When it is necessary to provide relief at different heights, it becomes necessary to provide a corresponding number of different finishing grindwheels.
Furthermore, for strongly cambered lenses that are beveled using a finishing grindwheel including a ridging groove, it is found that the bevel becomes partially pared away: it becomes thinner on the strongly cambered portions of the edge face of the lens. This thinning of the bevel degrades the quality and the accuracy with which the lens is mounted in the frame.
Finally, it can happen that the bevel engages in the bezel for the lens over only a fraction of the height of the bevel. In particular, the bottom portion of the flank of the bevel that faces towards the front of the lens is not covered by the rim of the frame. There thus exists a certain amount of space that remains visible between the rim of the frame end the front face of the lens, which is unattractive in appearance and tends to collect dirt.